1. Field of the Invention
The present invention mainly relates to an optical fiber for amplification that corresponds to tendency of being high-powered of a light amplifier or a fiber laser, an base material for optical fiber that is a material therefor, and a method for producing it.
2. Description of the Related Art
Recently, a light amplifier for system construction of DWDM (Dense Wavelength Division Multiplexing) has been strongly required to be high-power. Furthermore, there has been required high-power also in the field of fiber laser. In both cases, excitation light is introduced in quartz optical fiber wherein rare earth element is doped in the core, and emission of the rare earth element is used, and it is necessary for achievement of high-power to excite the core with the excitation light efficiently.
Recently, there was proposed a method called clad pumping for the purpose as described above (Japanese Patent Application Laid-open (kokai) No. 11-72629).
There is disclosed in Japanese Patent Application Laid-open (kokai) No. 11-52162 a double core type optical fiber for amplification wherein a second core is formed around a first core. In the optical fiber, for example, a heterogeneous part wherein a refractive index is different from the second core is formed, a part of the periphery of the second core is cut out, or the periphery is formed in polygonal shape, as scattering means for scattering the light that does not contribute to excitation of a signal light incident upon the first core among the excitation light incident upon the second core so that it may cross with the first core as possible. In order to produce such an optical fiber, the second core is formed around the first core by sooting to form a sintered glass, a part of the periphery is then cut out, and it is then coated with plastic with being wiredrawn to form a clad. An optical fiber for amplification is thereby produced.
In that case, the shape of the section of the second core (that corresponds to the first clad of the present invention) can be a deformed shape other than circle (D-shape, polygonal shape, drum shape), and quartz glass fine particles made of the same material as the second core can be formed by sooting thereon, to be deposited so that a shape of the section can be round. However, it was quite difficult to realize it for a problem in production thereof. Namely, breakages or cracks often occur on the surface of the base material in a process of cooling the porous quartz glass base material that is formed by sooting on the second core having a deformed shape, resulting in reduction of yield of the base material, and lowering of productivity. Accordingly, in most cases, polymer clad, for example those made of low refractive index polymer such as fluorinated acrylic acid, urethane acrylate, PMMA has been adopted as a clad, and the second core having a deformed shape has been coated with such plastics with being wiredrawn, to form a clad. However, there is a problem in production that not only wiredrawing of base material, but also a coating process, and a drying process are necessary in order to produce a polymer clad. Furthermore, there are problems in quality, such as bad adhesion property with the second core, difficulty in controlling a refractive index, low resistance to deterioration, low mechanical strength. Accordingly, there is a strong need for a quartz glass clad having excellent resistance to deterioration and stable properties.
The present invention has been accomplished to solve the above-mentioned problems, and a main object of the present invention is to provide a method for producing a base material for optical fiber wherein a lot of breakages or cracks on the surface of the base material can be prevented in a step of depositing porous glass fine particles for the second clad on a glass rod having a deformed first clad and cooling it, and base material for optical fiber having no defects, and an optical fiber having an efficient effect of being excited with excitation light.
In order to achieve the above-mentioned object, the present invention provides a method for producing a base material for optical fiber having a deformed first clad comprising at least a core, a first clad and a second clad comprising a step of deforming a shape of a section of the first clad so that it may have at least one linear part when the first clad is formed around the core, a step of depositing porous glass fine particles as the second clad made of the same material as that of the first clad on a glass rod having the deformed first clad to form a porous glass base material, and a step of forming the second clad having a round section by vitrifying it.
If the base material for optical fiber is produced according to the above-mentioned method, especially breakage and cracks that are liable to be formed in the step of forming the second clad by depositing porous glass fine particles on a glass rod having a deformed shape of a section can be prevented, so that a base material for an optical fiber having the second clad having no defect and the deformed first clad can be produced in high yield, and in high productivity. Furthermore, since the deformed shape is introduced into the section of the first clad, efficient incidence of the excitation light can be enabled, optical fibers wherein output light is increased can be produced. Moreover, since the deformed first clad and the second clad are made of the same material, the optical fiber having a stable refractive index distribution and excellent resistance to deterioration can be produced.
In that case, a length of the longest part of the deformed section of the glass rod having the first clad is preferably 15 mm or less.
As described above, if the length of the longest part of the deformed section of the glass rod having the deformed first clad that is to be a target is 15 mm or less in production of a porous glass base material by axially depositing porous glass fine particles on the glass rod having a deformed first clad, breakage and crack on the surface of the base material that are liable to occur in a cooling step after deposition can be prevented almost completely, so that the optical fiber having the second clad with no defects and the deformed first clad can be produced.
The present invention also provides a base material for optical fiber having a deformed first clad produced by the method described above. The optical fiber is a base material having a high quality wherein there is no defect such as breakage, crack or the like, particularly on the surface of the base material that is to be a front surface of the second clad. Since the deformed first clad having a deformed section with at least one linear part is formed, efficient incidence of excitation light is possible through the base material, so that an optical fiber that can emit high power light can be produced.
The present invention also provides a quartz base material for an optical fiber consisting of at least three refractive index area, namely a core, a deformed first clad and a second clad wherein section of the first clad has a deformed shape with at least one linear part and the second clad has a round section, and the main material of the first clad and the second clad are the same.
Using the base material for optical fiber having a deformed first clad with the above mentioned constitution, optical fiber wherein excitation light efficiently exciting the signal light incident on the core through the base material can be incident can be produced by wiredrawing. As a result, high-power can be achieved in a light amplifier or a fiber laser.
In that case, the shape of the section can be D-shape, a square shape or a drum shape.
When the shape of the section is the above mentioned shape, excitation efficiency of exciting light for exciting a signal light can be increased.
In that case, it is preferable that the refractive index of the first clad is distributed so that it may gradually decreases from the center to the periphery. Thereby, it is possible to make the excitation light concentrate at the core that is a center of the fiber, so that excitation efficiency can be further improved.
The present invention also provides an optical fiber having a deformed first clad produced by wiredrawing the base material for optical fiber described above. As described above, the optical fiber having a deformed first clad produced by wiredrawing the base material for optical fiber of the present invention is an optical fiber for light amplification that has no defects such as a crack and has high performance such as high excitation efficiency. Accordingly, it can easily achieve high-power in a light amplifier and a fiber laser or the like, and has high durability.
There were problems such as breakage or cracks occurs when a glass rod having a deformed section such as D-shape is axially deposited a porous glass fine particles made of the same material. However, according to the present invention, they can be solved. Especially, they could be solved by defining a length of the longest part of the deformed section of the glass rod having a deformed first clad that is to be a target to be 15 mm or less. Accordingly, high yield and improvement in productivity can be achieved in production of the base material for optical fiber having a deformed section in a first clad, and thereby cost can be reduced.
Furthermore, since there can be produced a base material for optical fiber having a deformed first clad with no defects such as crack or the like on the surface of the second clad and the deformed first clad which are made of the same material, excitation light can be efficiently incident on a signal light incident on the core, and output light can be increased as a result, in the optical fiber produced therefrom. Thereby high-power in a light amplifier and a fiber laser can be achieved.